[Lldb-commits] [lldb] [lldb][AArch64] Invalidate SVG prior to reconfiguring ZA regdef (PR #66768)

[David Spickett via lldb-commits]

https://github.com/DavidSpickett created https://github.com/llvm/llvm-project/pull/66768

This fixes a bug where writing vg during streaming mode could prevent you reading za directly afterwards.

vg is invalided when the write happens. SVE is reconfigured by reading back the new value from the remote. So far so good.

For SME, we read SVG which in some situations appeared to remain valid but have a value of 0 (perhaps it had never been read). This meant lldb expected the size of za to be 0 bytes and rejected the remote's response.

To fix this, invalidate svg before reconfiguring. This ensures that the value used is the latest one from the remote.

The bug may depend on timing, I could not find a consistent way to trigger it. I originally found it when checking whether za is disabled after a vg change, so I've added checks for that to TestZAThreadedDynamic.

Differential Revision: https://reviews.llvm.org/D158514

>From f4915dd9acd3b819a9745dee1d410b465edd57c4 Mon Sep 17 00:00:00 2001
From: David Spickett <david.spickett at linaro.org>
Date: Tue, 22 Aug 2023 14:42:35 +0100
Subject: [PATCH] [lldb][AArch64] Invalidate SVG prior to reconfiguring ZA
 regdef

This fixes a bug where writing vg during streaming mode
could prevent you reading za directly afterwards.

vg is invalided when the write happens. SVE is reconfigured
by reading back the new value from the remote. So far so good.

For SME, we read SVG which in some situations appeared to remain
valid but have a value of 0 (perhaps it had never been read).
This meant lldb expected the size of za to be 0 bytes and
rejected the remote's response.

To fix this, invalidate svg before reconfiguring. This ensures
that the value used is the latest one from the remote.

The bug may depend on timing, I could not find a consistent way
to trigger it. I originally found it when checking whether za
is disabled after a vg change, so I've added checks for that
to TestZAThreadedDynamic.

Differential Revision: https://reviews.llvm.org/D158514
---
 .../Plugins/Process/gdb-remote/GDBRemoteRegisterContext.cpp  | 5 +++++
 .../za_dynamic_resize/TestZAThreadedDynamic.py               | 2 ++
 2 files changed, 7 insertions(+)

diff --git a/lldb/source/Plugins/Process/gdb-remote/GDBRemoteRegisterContext.cpp b/lldb/source/Plugins/Process/gdb-remote/GDBRemoteRegisterContext.cpp
index 16dde28f6ee5396..13365dc312f5b66 100644
--- a/lldb/source/Plugins/Process/gdb-remote/GDBRemoteRegisterContext.cpp
+++ b/lldb/source/Plugins/Process/gdb-remote/GDBRemoteRegisterContext.cpp
@@ -781,6 +781,11 @@ void GDBRemoteRegisterContext::AArch64Reconfigure() {
   std::optional<uint64_t> svg_reg_value;
   const RegisterInfo *svg_reg_info = m_reg_info_sp->GetRegisterInfo("svg");
   if (svg_reg_info) {
+    // When vg is written it is automatically made invalid. Writing vg will also
+    // change svg if we're in streaming mode but it will not be made invalid
+    // so do this manually so the following read gets the latest svg value.
+    SetRegisterIsValid(svg_reg_info, false);
+
     uint32_t svg_reg_num = svg_reg_info->kinds[eRegisterKindLLDB];
     uint64_t reg_value = ReadRegisterAsUnsigned(svg_reg_num, fail_value);
     if (reg_value != fail_value && reg_value <= 32)
diff --git a/lldb/test/API/commands/register/register/aarch64_za_register/za_dynamic_resize/TestZAThreadedDynamic.py b/lldb/test/API/commands/register/register/aarch64_za_register/za_dynamic_resize/TestZAThreadedDynamic.py
index 65d1071c26b2a34..d2a26ce71bde1d8 100644
--- a/lldb/test/API/commands/register/register/aarch64_za_register/za_dynamic_resize/TestZAThreadedDynamic.py
+++ b/lldb/test/API/commands/register/register/aarch64_za_register/za_dynamic_resize/TestZAThreadedDynamic.py
@@ -125,11 +125,13 @@ def za_test_impl(self, enable_za):
                 self.runCmd("thread select %d" % (idx + 1))
                 self.check_za_register(4, 2)
                 self.runCmd("register write vg 2")
+                self.check_disabled_za_register(2)
 
             elif stopped_at_line_number == thY_break_line1:
                 self.runCmd("thread select %d" % (idx + 1))
                 self.check_za_register(2, 3)
                 self.runCmd("register write vg 4")
+                self.check_disabled_za_register(4)
 
         self.runCmd("thread continue 2")
         self.runCmd("thread continue 3")